The present invention relates to a piezoelectric oscillation circuit; in particular, to an inexpensive piezoelectric oscillation circuit that has excellent aging characteristics.
Crystal oscillators that have excellent frequency stability (hereafter referred to as xe2x80x9caging characteristicsxe2x80x9d) despite years of changes generally comprise an AGC circuit such as that shown in FIG. 4 in order to operate a crystal vibrator at low excitation levels, to restrain the driving stress exerted upon on a crystal vibrator.
FIG. 4 is a circuit diagram of a crystal oscillator comprising a conventional AGC circuit.
In crystal oscillator 100 shown in the figure, output from an oscillation circuit 101 enclosed by a broken line is supplied to a buffer circuit 103 in the next stage via a capacitor 102; output from buffer circuit 103 is applied to an AGC circuit 104 enclosed by -a broken line; and a signal at an anode of a diode 116 which corresponds to an output from AGC circuit 104 is returned to oscillation circuit 101.
Oscillation circuit 101 is of a general Colpitts type: a crystal vibrator 106 with one end grounded via a capacitor 105 has the other end connected to a base of a transistor 107, a series circuit composed of capacitors 108 and 109 as well as a resistor 110 are connected between the base and the ground, and a connection midpoint of the series circuit composed of capacitors 108 and 109 is connected to an emitter of transistor 107, the emitter being grounded via a resistor 111.
The base of transistor 107 is connected to the output of AGC circuit 104 via a resistor 112, and a collector of transistor 107 is connected to a power supply Vcc via a resistor 113.
The AGC circuit 104 is comprised of resistors 114, 120, 121, 122, capacitors 115, 118, 119, diodes 116 and 117. A cathode end of the diode 116 and an anode end of the diode 117 connect to an output end of buffer circuit 103 via a series circuit of resistor 114 and capacitor 115. The other ends of the diodes are grounded via capacitor 118 and 119, respectively and are connected together via a resistor 120. The anode end of diode 116 is connected to the base of transistor 107 via resistor 112 included in oscillation circuit 101. Additionally, a series circuit composed of resistors 121 and 122 is connected between the power supply Vcc and the ground and has its connection midpoint connected to a cathode of diode 117.
Buffer circuit 103 is connected to an output of the oscillator via a series circuit composed of a resistor 123 and a capacitor 124.
The operation of the crystal oscillator will be described below. The oscillation circuit 101 is of the general Colpitts type; since the particulars of this circuit are well known, a description is omitted.
When an excitation signal output from oscillation circuit 101 is applied to AGC circuit 104 via buffer 103, a plus half cycle of the signal is directed to the ground via diode 117, and a minus half cycle of the signal is directed to the ground via diode 116. Accordingly, a current A is generated between terminals of resistor 120 due to the occurrence of such a voltage that provides a low voltage at the terminal connected to diode 116.
In addition to current A, a current B determined on the basis of a voltage generated at a connection midpoint between resistor 121 and resistor 122 is supplied to resistor 120. As a result, a total current obtained by subtracting current B from current A is supplied to a base current flowing through transistor 107, and the base current determined by the total current causes the crystal vibrator 106 to generate an excitation signal of a desired fixed level.
In the above operational state, if the level of the excitation signal from crystal vibrator 106 increases, the level of the excitation signal supplied to AGC circuit 104 increases and the voltage across resistor 120 rises to increase the value of current A, thereby reducing the total current (current Bxe2x80x94current A) and the base current of transistor 107.
The decrease in the base current of transistor 107 reduces the gain of transistor 107, reducing the output level (the collector output level of transistor 107) of oscillation circuit 101 and thus reducing the emitter voltage (AC component) level of transistor 107. Consequently, the level of a signal generated between the terminals of capacitor 109 falls, reducing the level of the excitation signal from crystal vibrator 106.
If the level of the excitation signal from crystal vibrator 106 increases, then an operation that is the reverse of the above-described operation occurs. A description of this operation is omitted.
The above operation is repeated to have the level of the excitation signal from the crystal vibrator 106 being stable to the desired value.
However, since the electronic parts (including the crystal vibrator) constituting the oscillation circuit and AGC circuit have varying characteristics, the circuit must be configured to obtain a low and desired excitation levelxe2x80x94that is, an optimal excitation levelxe2x80x94taking into account the characteristics of the individual parts. This prevents cost-efficient production of the crystal oscillator.
It is an object of the present invention to solve the above problems to provide an inexpensive crystal oscillator having excellent aging characteristics.
To resolve this problem, the present invention provides a piezoelectric oscillator comprising a Colpitts circuit including a piezoelectric vibrator and a load capacitance comprising at least a capacitor in the oscillating loop, and a limiter circuit, wherein an excitation signal from the piezoelectric vibrator is supplied to the limiter circuit. Based on variations in excitation signal, the limiter At controls an inter-terminal impedance of said capacitor which is connected to said limiter and thus clips (the sill level outside the range of a fixed is eliminated) the level of the excitation signal.
In this configuration, the limiter circuit comprises a series circuit comprising two diodes connected in series so that a forward current of the diodes flows from a power supply toward a ground, and a capacitor connected in parallel win the series circuit, with a connection midpoint between the two diodes connected to one end of the capacitor.
Moreover, the capacitor is inserted and connected between the piezoelectric vibrator and the ground; the capacitor includes two divided capacitors connected in series; the limiter circuit comprises a series circuit comprising two diodes connected in series so that a forward current of the diodes flows from a power supply toward a ground; and a capacitor connected in parallel with the series circuit, and a connection midpoint between the two divided capacitors and a connection midpoint between the two diodes are connected together via a capacitor.
Moreover, the present invention provides a Colpitts circuit comprising a transistor circuit having a resistance element connected to an appropriately biased emitter, a piezoelectric vibrator inserted between a base and a ground of he transistor circuit, and two divided capacitors, one of which is connected between the base and emitter of the transistor, with the other connected in parallel with the emitter resistor, wherein a resistor of a desired value and two diodes are inserted in series between a power supply and the ground, the Colpitts circuit includes a capacitor for earthing the diode series circuit toward a high frequency, and the two diode connection points and the emitter of the transistor are connected to obtain an alternate current.